All materials change dimension with enviromental temperature changes in accordance with their individual coefficients of thermal expansion. Thus, in various constructions this phenomena of contraction and expansion must be compensated for to a degree acceptable for the particular construction undertaken.
Familiar examples of this compensation are seen every day in the construction of highways where room for expansion is provided between lengths of pavement and in bridge or building construction where room for expansion is provided between structural elements such as metal bars and struts. Changes in dimension of other materials such as concrete, plastic and other building compositions also must to some extent be taken into account.
In some instances, for example, in precision structures, thermal expansion or contraction must be compensated for within the individual structural elements, themselves. Precision structures such as optical instruments where precise positioning between optical elements must be unaffected by dimensional changes due to temperature variations require structural members such as struts which have zero or near zero expansivity.
The attainment of a zero, near-zero or a specific finite expansivity in a graphite-epoxy structural member is a function of a multitude of design, material and process variables. These variables include such things as the raw material used and their elastic and thermoelastic properties, epoxy bleed-rate and final percentage of epoxy present after curing, layup geometry and curing pressure, temperature and rheology. As a consequence of these factors, the attained expansivities generally exhibit scatter on a part to part basis of the order of .+-.0.05X10.sup.-6 in/in/F.degree.. Such an expansivity factor is not acceptable in structures such as large telescope structures where the effective coefficient of thermal expansion tolerable can often be no more than .+-.0.01X10.sup.-6 in/in/F.degree.. To accomodate this requirement the elements comprising the above structures must be made in a way that all the process variables are compensated for.